1. Field of the invention
The present invention relates to a release sheet, and more particularly, to a release sheet comprising a support and a release layer provided thereon of which release characteristics are controlled and which bonds fast to the support.
2. Description of the Related Art
A release sheet is composed of a sheet-like support such as aqueous coated paper, laminated paper, synthetic paper, non-woven cloth, plastic film, metal foil and the like and a release layer provided on the sheet-like support. A release sheet is used as a release sheet for adhesive products such as pressure sensitive adhesive tapes, labels and the like; it is also used in manufacturing synthetic leather, prepreg, flooring material, various molded articles and the like.
Recently, a low temperature curing type technique has been developed to manufacture a release sheet, wherein a non-solvent type silicone resin, in particular, one having functional groups polymerizable by irradiation with ultraviolet ray or electron beam, is applied on a support, and then cured by being subjected to ultraviolet ray or electron beam.
This technique was relatively successful in eliminating certain quality defects like curling, wrinkles and pinholes of a release sheet attributable to a high heat processing of a conventional technique making use of a heat curing silicone resin.
However, this low temperature curing type technique was not free of problems. Use of said non-solvent type silicone resin necessitated application of an aqueous coating layer or a polyethylene primer layer on a support in order to block permeation of said non-solvent type silicone resin into the support, and such an aqueous coating layer or polyethylene primer layer in turn tends to weaken bond between the support and release layer; bonding within a release layer tends to be weak, resulting in an inner exfoliation (so-called "rub-off"); moreover, the silicone resin transfers to a tacky layer when it is laid on the release layer.
In order to solve the above-mentioned problems, there is proposed a method in which a polyacrylate ester having two, three or 4 functional groups is added to an electron beam curing silicone resin (Japanese Patent Publication No. 57096/1982). However, according to this method, when the resin dispersion is allowed to stand, it becomes separated into different phases and therefore, it has to be dispersed again right before coating. If such redispersion is omitted, the expected release characteristic can not be obtained and moreover, more dose of electron beam irradiation is required for curing.
Further, there are proposed a method of increasing the concentration of acryl or methacryl group to be introduced into an electron beam curable silicone resin so as to enhance bond between the support and the release layer and to inhibit transfer of the silicone resin to the adhesive layer when it is laid (Japanese Patent Publication No. 233162/1985), and a method of providing as an anchor layer an ultraviolet light or electron beam curable resin bonding fast to a support between an electron beam curable silicone resin layer and a support (Japanese Patent Application Laid-Open No. 80525/1989).
Even if such methods are employed, the bond between the silicone resin layer and the anchor layer is still insufficient, rub-off of the silicone resin layer still occurs, and moreover this silicone resin still transfers to a tacky layer when it is laid on the silicone layer. Moreover, in the case of a release sheet used as a processing paper for producing a synthetic leather, a prepreg and the like, there is a problem that a commercially available electron beam curable silicone resin exhibits only too poor release property.